This invention relates generally to game balls, and more particularly to a game ball, such as a golf ball, having a cover formed predominately of terpolymer resin materials.
For many years balata was the preferred material for golf ball covers. Later, polyethylene was proposed for use as a golf ball cover material but was generally deemed highly inferior to balata in imparting playability and durability characteristics to the ball due to its brittleness and high hardness, and thus never became a commercially successful golf ball cover material.
Balata golf ball covers have now been replaced to a great extent by ionomer resin cover materials. Ionomer resins or xe2x80x9cionomersxe2x80x9d are polymers having interchain ionic bonding. Ionomers typically comprise an ethylenically unsaturated carboxylic acid with a portion of the carboxylic acid groups neutralized by cations. The cations serve as cross-linking agents, as they are ionically bonded to carboxylic acid groups in adjacent copolymer chains. Instead of having thermally irreversible covalent bonding, ionomers have thermolabile cross-linking in which cations become part of the chemical structure of the ionomer upon cross-linking, and these cross-links are reversible. For purposes of this application, this type of cross-linking is referred to as ionic cross-linking. Ionomers may comprise two monomers, three monomers (a terpolymer) or more monomers.
Hard ionomers show increased game ball cover durability. The advantages of hard ionomer game ball covers are offset to some degree by the decrease produced in playability. This is because the durable ionomer resins tend to be very hard and thus lack the preferred degree of softness to impart the spin necessary to optimally control the ball in flight.
Terpolymers typically include monomers of an olefin, an xcex1, xcex2-ethylenically unsaturated carboxylic acid and an unsaturated acrylate ester. The carboxylic acid may be partially or fully neutralized with cations to form an ionomer or may be unneutralized. The unsaturated acrylate ester monomer can function to soften the resulting terpolymer. Non-ionic terpolymers, that is terpolymers in which the carboxylic acid groups have not been neutralized by cations, tend to be softer than similar neutralized materials. U.S. Pat. No. 5,902,855 discloses the use of unneutralized terpolymer resins to form golf ball covers. One drawback of unneutralized terpolymer resins is that the softer covers formed therefrom are somewhat more prone to scuffing and/or cutting, particularly when the game ball is a golf ball being hit with irons that have sharp grooves. It would be useful to develop a golf ball with a softer cover having an improved resistance to at least one of scuffing and cutting.
An object of the invention is to provide an unneutralized terpolymer resin game ball cover having improved scuff resistance and/or cut resistance.
Another object of the invention is to provide a method for imparting improved scuff resistance and/or cut resistance to a game ball cover comprising unneutralized terpolymer resins.
Yet another object of the invention is to provide a golf ball with a soft cover which is well suited for use with golf club irons having sharp grooves.
Yet another object of the invention is to provide a method of forming a golf ball with a soft cover which has excellent scuff resistance and/or cut resistance.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
The invention in one embodiment is a game ball having a cover comprising more than 90 weight % of one or more acrylate ester-containing terpolymer resins. The one or more acrylate ester-containing terpolymer resins are each preferably formed from (a) an olefin having 2 to 8 carbon atoms, (b) an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms, and (c) an acid which includes at least one member selected from the group consisting of xcex1, xcex2-ethylenically unsaturated mono-or dicarboxylic acids. The olefin preferably is an alpha olefin, and the acid preferably is acrylic acid. Preferably, the game ball has a rough or dimpled surface. In one aspect of the invention, the game ball is a golf ball.
In a particularly preferred form of the invention, a golf ball cover comprises one or more acrylate ester-containing terpolymer resins. Each of the acrylate ester-containing terpolymer resins preferably comprises ethylene, at least one acid selected from the group consisting of acrylic acid, maleic acid, fumaric acid, itaconic acid, methacrylic acid, and half-esters of maleic, fumaric and itaconic acids, and at least one co-monomer selected from the group consisting of methyl, ethyl, n-propyl, n-butyl, n-octyl, 2-ethylhexyl, and 2-methoxyethyl-1-acrylates. The cover preferably contains at least 95 weight % terpolymer resin.
The terpolymer resin has a sufficient degree of covalent cross-linking to impart to the golf ball cover improved resistance to at least one of scuffing and cutting. In one aspect of the invention the covalent cross-linking comprises radiation-induced covalent cross-linking.
In a preferred form of the invention, the degree of radiation induced covalent cross-linking is appropriate to impart to the cover a Shore D hardness which is no more than about 25% greater, more preferably no more than about 10% greater and most preferably no more than about 5% greater, than the Shore D hardness of a cover having an identical composition but which does not include a substantial degree of radiation induced covalent cross-linking.
Another aspect of the invention is a method of treating a game ball. The method comprises the steps of obtaining a game ball having a cover comprising at least one unneutralized terpolymer resin, and irradiating the terpolymer resin in the cover under conditions appropriate to covalently cross-link the resin in order to increase the resistance of the cover to at least one of scuffing and cutting without substantially impairing other playability characteristics of the ball. Preferably, the game ball has a dimpled surface. In a particularly preferred form of the invention, the game ball is a golf ball.
The game ball cover is subjected to irradiation at a dosage in the range of 10 to 200 kiloGrays (kGy) or more. It is anticipated that game balls irradiated at dosages greater than 200 kGy would show continued improvement in certain properties. The game ball cover preferably is irradiated prior to application of a top coat over a cover. The method of the invention preferably further includes the step of applying a top coat over the cover before or after irradiation.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements exemplified in the following detailed disclosure.